THERMAL PROPERTIES,MICROSTRUCTURE AND HYDROGEN STORAGE PROPERTIES OF PELLETIZED MAGNESIUM+2WT% MULTI-WALLED CARBON NANOTUBES COMPOSITES

Magnesium is considered as one of the leading candidates for solid state hydrogen storage because of its high hydrogen storage capacity and abundance in the Earth crust. However, slow hydrogen absorbtion/desorbtion kinetics, high hydride formation enthalpy and poor thermal conductivity of hydride phase are the main drawbacks that prevent its use in hydrogen storage. The issues are interconnected: low thermal conductivity slows down the heat flow that influences the rate of hydrogen absorption/desorption.

Among various carbonaceous additives to Mg, carbon nanotubes exhibit one of the best catalytic effects. They are also excellent thermal conductors: thermal conductivity of the individual multi-wall carbon nanotube (MWCNT) can reach up to ~3000 W/mK at the ambient temperature [1]. However, the mass-produced MWCNTs are randomly oriented and entangled inside the agglomerates. Numerous junctions between the nanotubes segments inside these agglomerates turn them into thermal insulator rather than conductors. Ball-milling is a processing method that can easily disperse the undesired agglomerates, while tuning morphologies and properties of the MWCNTs and their derivatives.

In this work we synthesized solid porous pellets of Mg co-milled with 2 wt.% of MWCNTs. We studied the correlation between the morphology, hydrogenation kinetics and thermal properties of both as-synthesized and hydrogenated pellets. It was found that although prolonged ball-milling leads to a partial destruction of the MWCNTs, it has a positive effect on hydrogen absorption/desorption kinetics. The role of carbonaceous derivatives of the MWCNTs in enhancing the hydrogen storage properties and thermal conductivity of the pelletized composites in fully hydrogenated state is discussed.

This work was supported by the Israel Strategic Alternative Energy Foundation (I-SAEF).

1. P. Kim, L. Shi, A. Majumdar, and P. L. McEuen, Phys. Rev. Lett. 2001, 87, 215502.